Solvent effects on the photodissociation of formic acid: a theoretical study

Photodissociation of aqueous formic acid has been investigated with the CASSCF, DFT, and MR-CI methods. Solvent effects are considered as a combination of the hydrogen-bonding interaction from explicit H2O molecules and the effects from the bulk surrounding H2O molecules using the polarizable continuum model. It is found that the hydrogen-bonding effect from the explicit water in the complex is the major factor to influence properties of aqueous formic acid, while the bulk surrounding H2O molecules has a noticeable influence on the structures of the complex. The direct C-O bond fission along the S1 pathway is predicted to be an important channel upon photolysis of aqueous formic acid at 200 nm, which is consistent with experimental observation that aqueous formic acid dissociates predominantly into fragments of HCO and OH. The existence of a dark channel upon photolysis of aqueous formic acid at 200 nm is assigned as fast relaxation from the S1 Franck-Condon geometry to the T1/S1 intersection and subsequent S1-->T1 intersystem crossing process. S1-->S0 internal conversion followed by molecular elimination to CO+H2O is the most probable primary process for formation of carbon monoxide, which was observed with considerable yield upon photolysis of aqueous formic acid at 253.7 nm.     

Experimental and theoretical study of substituent effects of iodonitrobenzenes

The electronic structures and substituent effects of o-, m-, and p-iodonitrobenzene have been studied by ultraviolet photoelectron spectroscopy (UPS). The observed bands were interpreted on the basis of empirical arguments and theoretical calculations. The analysis of electronic effects of the donor/acceptor substituent groups is essential for the reliable assignment of the observed photoelectron spectra. The investigation of pi- and n-orbital ionization potentials enabled us to describe the correlation between substituent effects and the relative reactivities of the iodonitrobenzenes. It was found that the energy order of the pi(2) and n(II) parallel orbitals is reversed as a result of the combined influence of the electron-withdrawing nitro group and the electron-donating iodine atom. Distinct changes of the pi and n bands occur in o-iodonitrobenzene. This characteristic depends on the conjugation between the pi orbitals of the benzene ring and the nitro group and the interaction of in-plane lone pairs of iodine and one of the oxygen atoms of the nitro group in the adjacent position. This might contribute to the high reactivity of o-iodonitrobenzene in a number of reactions.     

Solvent and substituent effects on the photochemistry of norbornadiene-diarylacetylene Pauson-Khand adducts

The photochemistry of Pauson-Khand cycloadducts of norbornadiene with a series of bis-aryl alkynes has been studied. Two types of photochemical transformation take place: photorearrangement to tricyclic ketones or photochemical 6π electrocyclization. High selectivity levels have been attained for each pathway, controlled by the polarity of the solvent, irradiation wavelength, and presence (or absence) of oxygen.     

Solvent effects on the thioamide rotational barrier: an experimental and theoretical study.

The solvent effect on the C-N rotational barriers of N,N-dimethylthioformamide (DMTF) and N,N-dimethylthioacetamide (DMTA) has been investigated using ab initio theory and NMR spectroscopy. Selective inversion recovery NMR experiments were used to measure rotational barriers in a series of solvents. These data are compared to ab initio results at the G2(MP2) theoretical level. The latter are corrected for large amplitude vibrational motions to give differences in free energy. The calculated gas phase barriers are in very good agreement with the experimental values. Solvation effects were calculated using reaction field theory. This approach has been found to give barriers that are in good agreement with experiment for many aprotic, nonaromatic solvents that do not engage in specific interactions with the solute molecules. The calculated solution-phase barriers for the thioamides using the above solvents are also in good agreement with the observed barriers. The solvent effect on the thioamide rotational barrier is larger than that for the amides because the thioamides have a larger ground-state dipole moment, and there is a larger change in dipole moment with increasing solvent polarity. The transition-state dipole moments for the amides and thioamides are relatively similar. The origin of the C-N rotational barrier and its relation to the concept of amide "resonance" is examined.     

Remote substituent effects on the oxymercuration of 2-substituted norbornenes: an experimental and theoretical study.

The effect of a remote substituent on regioselectivity in the oxymercuration of 2-substituted norbornenes has been investigated experimentally and theoretically using density functional theory (DFT). Regioselectivities of 1:1 to 14:1 were observed with various 2-substituted norbornenes. Exo-2-substituted norbornenes always gave greater regioselectivities compared to the corresponding endo-2-substituted norbornenes. The effects of solvents on the regioselectivity have also been examined, and ethereal solvents were found to be the best choice giving the optimal yield and regioselectivity. The relative rate of oxymercuration was estimated by competition experiments. The least reactive substrate (X = OAc) gave the highest regioselectivity. According to DFT predictions, the increased difference between the reaction barriers that results in the greater regioselectivity is correlated directly with the larger polarity of the C=C double bond, which is attacked by the mercury and oxygen. A number of stable exo and endo conformers were predicted. All exo conformers show the same polarity of the double bond, while some endo conformers have a reversal of this polarity. All the conformers except those with the OAc substituent are very close in energy and thus should react. The existence of a mixture of endo conformers with the C=C double bond of opposite polarity clearly explains a decrease in regioselectivity for the endo species. The origin of the greatest regioselectivity for the OAc-2-norbornenes lies in the fact that the conformer with the largest polarity is notably lower in energy than others due to an internal C-H-O hydrogen bond.     

Solvent effects on tamoxifen molecule interacting with a single-walled carbon nanotube: a theoretical NMR study

Quantum chemical calculations of the electronic structure of tamoxifen molecule interacting with an open end of a single-walled carbon nanotube (SWCNT) were carried out and the effects of solvents (water, methanol, DMSO, acetone) on the ¹H, ¹³C, ¹⁵N, and ¹⁷O NMR parameters were studied by the GIAO-HF/STO-3G, GIAO-HF/3-21G, and GIAO/B1LYP/3-21G methods using the GAUSSIAN-98 program. The largest σ_iso value was obtained for acetone, whereas the smallest one for water. The opposite trend was obtained for the shielding asymmetry η. According to calculations, atoms at interaction site bear negative charges. The O(43) and N(38) atoms produce negative charge because they have high electron affinities. The dipole moment of tamoxifen molecule in different solvents increases with increasing the dielectric constant of the solvent. The largest dipole moment value was obtained for water by the B1LYP/3-21G method.     

Solvent and substituent effects on spectroscopical changes of some diazoaminobenzene derivatives

A series of diazoaminobenzene derivatives (seven) in which the substituents have a wide range of electronic characters are set out to understand the involvement of the substituent identity in controlling the changes in their electronic absorption spectra. The interactions between the diazoamino group and the different groups account for some spectral shifts. The UV-vis spectrum of each compound is measured in several solvents with wide variations of solvent polarity parameters to examine the role of the chemistry of the solvent in these spectroscopical changes. The electronic transitions are assigned and the solvent induced spectral shifts are analyzed in relation to the different solute-solvent interaction mechanisms using computational chemistry. The regression analysis is applied for correlating the different parameters. The results help to assign the solute-solvent interactions and the solvatochromic potential of the investigated compounds. It is concluded that the electronic character of the substituent and the chemical nature of the solvent are the major factors for the observed solvatochromism.     

A theoretical study of substituent effects on allylic ion and ion pair SN2 reactions

An ab initio study of ionic and ion pair displacement reactions involving allylic systems has been carried out at the RHF/6-31+G* level. The geometries and natural charges show the absence of conjugative stabilization in the ionic transition states, thus differing from traditional explanations. The high reactivity of allyl halides is explained by electrostatic polarization of the double bond. Substituent effects were also studied; in general, electron-withdrawing groups lower the barriers of the ionic S(N)2 reactions but increase the barriers of the ion pair reactions. The allylic reactions are compared with related benzylic systems. Hammett correlations give rho of opposite sign for the ionic and ion pair displacement reactions, in agreement with some experimental results.     

Solvent effects on heavy atom nuclear spin-spin coupling constants: a theoretical study of Hg-C and Pt-P couplings.

The computation of indirect nuclear spin-spin coupling constants, based on the relativistic two-component zeroth order regular approximate Hamiltonian, has been recently implemented by us into the Amsterdam Density Functional program. Applications of the code for the calculation of one-bond metal-ligand couplings of coordinatively unsaturated compounds containing (195)Pt and (199)Hg, including spin-orbit coupling or coordination effects by solvent molecules, show that relativistic density functional calculations are able to reproduce the experimental findings with good accuracy for the systems under investigation. Spin-orbit effects are rather small for these cases, while coordination of the heavy atoms by solvent molecules has a great impact on the calculated couplings. Experimental trends for different solvents are reproduced. An orbital-based analysis of the solvent effect is presented. The scalar relativistic increase of the coupling constants is of the same order of magnitude as the nonrelativistically obtained values, making a relativistic treatment essential for obtaining quantitatively correct results. Solvent effects can be of similar importance.     

Solvent and substituent effects on the conversion of 4-methoxypyridines to N-methyl-4-pyridones

In the reaction of 4-methoxypyridine derivatives with alkyl iodides in the presence or absence of solvent, not only the pyridinium ions but also the related 1-methylpyridones are produced. The presence of solvent favors the formation of the 1-methylpyridone. Electron withdrawing groups on the pyridine ring also favor this conversion. A possible mechanism is presented.     

Solvent effects on the 9-hydroxymethylanthracene + N-ethylmaleimide Diels-Alder reaction. A theoretical study

[structure: see text] The origin of the high reaction rates of the 9-hydroxymethylanthracene + N-ethylmaleimide Diels-Alder reaction in fluorous solvents and supercritical carbon dioxide is analyzed through a combination of regression analyses and theoretical calculations. Both approaches allow the solvent effects on the activation barrier (a decrease by solvophobic interactions, an increase by dipolarity-polarizability) to be attributed to the existence of a hydrogen bond between the two reactants in the transition state, refuting a previous hypothesis based on strong solvophobic interactions.     

Solvent effects on the geometry,electronic structure,and bonding style of Zn(N5)2: A theoretical study

Nitrogen-rich compounds involving the cyclo-pentazole anion (cyclo-N₅⁻) have attracted extensive attention due to higher energy release and environmental friendliness than traditional high energy density materials (HEDMs). However, the synthesis of stable HEDMs with cyclo-N₅⁻ is still a challenge. In this study, the effect of nine solvents on the geometrical and electronic structures and solvation energies of Zn(N₅)₂, one of the recently synthesized nitrogen-rich compounds, was studied using the density functional theory and the polarized continuum model. The results indicated an increase in the stability of Zn(N₅)₂ in the solution phase compared to the vacuum phase, and the stability of Zn(N₅)₂ increases with increasing dielectric constants. The energy gap of frontier molecular orbitals and the absolute value of total energy in water are the largest, revealing that Zn(N₅)₂ is more stable in water than in other solvents. To understand the stabilization mechanism of Zn(N₅)₂ by water, further studies were performed with the natural bond orbital (NBO) analysis and the quantum theory of atoms in molecules (QTAIM) analysis using the explicit solvent model. The charge transfer and the hydrogen bonds are observed between Zn(N₅)₂ and water, which are beneficial to improvement of the stability of Zn(N₅)₂. This may indicate the solvents that have strong interactions with the cyclo-N₅⁻ candidate may improve the possibility of success of synthesis.     

Remote substituent effects in ruthenium-catalyzed [2+2] cycloadditions: an experimental and theoretical study

The effects of a remote substituent on the regioselectivity of ruthenium-catalyzed [2+2] cycloadditions of 2-substituted norbornenes with alkynes have been investigated experimentally and theoretically using density functional theory. Most of the cycloadditions occurred smoothly at room temperature, giving the exo cycloadducts in excellent yields. Regioselectivities of 1.2:1 to 15:1 were observed with various substituents on the C-2 position of the norbornenes. Exo-C-2-substituents usually showed greater remote substituent effects on the regioselectivities of the cycloadditions than the corresponding endo-C-2-substituents. The regioselectivity of the cycloadditions with C-2 substituents containing an exocyclic double bond (sp2 hybridized carbon at C-2) are much higher than the cycloadditions with the exo and endo 2-substituted norbornenes. Theoretical studies predicted the same trends as experiment and matched the experimental product ratios well. The nature of the regioselectivity in this reaction is discussed. Different strengths of the pi(C5-C6)-->pi(C2-Y) or pi(C5-C6)-->sigma(C2-Y) orbital interactions in 2-substituted norbornenes result in different degrees of C5-C6 double bond polarization. Stronger C5-C6 polarization will increase the difference in the activation energies between the major and minor pathways and thus lead to greater regioselectivities.     

Solvent impact on the planarity and aromaticity of free and monohydrated zinc phthalocyanine: a theoretical study

A theoretical investigation on the planarity of molecular structure of zinc phthalocyanine (ZnPc) and its aromaticity has been performed using B3LYP and M06-2X density functionals combined with selected Pople-type basis sets. The effect of the applied calculation method on the optimized structure of ZnPc and ZnPc???H₂O, both in the gas phase and in the polar solvent, was analyzed. To quantify the aromaticity of the ZnPc and ZnPc???H₂O complexes, both the geometric and magnetic criteria, i.e., Harmonic Oscillator Model of Aromaticity (HOMA) index and the nucleus-independent chemical shift (NICS) values at the centers or 1 Å above the centers of structural subunits, were calculated. The energies of highest energy occupied molecular orbital (HOMO) and lowest energy unoccupied molecular orbital (LUMO) and energy gaps were also estimated. The results show that the free ZnPC molecule is flat in the gas phase and nonplanar in the polar environments (DMSO and water). ZnPC???H₂O is nonpolar in the gas phase and polar solvent which is in agreement with recent X-ray reports. Both HOMA and NICS indexes indicate the presence of highly aromatic macrocycle and benzene rings while these parameters for pyrrolic ring are significantly smaller than in free pyrrole. The presence of polar solvents practically does not change aromaticity of the ring subunits of the studied compounds.     

Solvent effects on IR and VCD spectra of natural products: an experimental and theoretical VCD study of pulegone

The VCD spectra of pulegone, dissolved in CDCl3, CD2Cl2 and CS2 have been recorded in the frequency range from 1000 to 3000 cm(-1). The assignment of the absolute configuration was performed by comparing the experimental data with theoretical spectra computed at the B3LYP/6-311+G(d,p) level. Analysis of the agreement in several spectral regions revealed significant shortcomings when comparing with vacuum calculations. It is shown that the agreement improves when the solvent effects are taken into account by a continuum model. For the measurements in CDCl3 and CD2Cl2 further improvement was found when considering explicitly 1 : 1 complexes between a pulegone and a CDCl3 or CD2Cl2 solvent molecule in vacuo, while the best agreement was obtained when embedding these in a continuum model. The presence of the chiral solute was found to induce a VCD active C-D stretch band which could be modeled also at ab initio level.     

Solvent effects on the excited state properties of 2-aminopurine—a theoretical study by the ONIOM and Supramolecular method

In this paper, the micro-solvated effects on the lowest-energy vertical transition state and adiabatic excited states of 2-aminopurine (2Ap) were studied by Supramolecular method (B3LYP/6-31++G(d)) and ONIOM (B3LYP/6-31++G(d):PM3) method. The results are as follows: (1) In 2Ap molecule surrounded by six water molecules the pyramidalization of amino group in 2Ap almost disappears, and the hex-atomic ring is obviously buckled. The adiabatic lowest-energy valence excitation of gaseous 2Ap also causes the disappearance of amino pyramidalization. (2) The energy for lowest-energy singlet π→π^* vertical transition in water is predicted as 3.99 and 4.29 eV by Supramolecular and ONIOM method, respectively. Both values are in good agreement with the reported experimental result, 4.11 eV. The energy for the second lowest-energy n→π^* transition, 4.72 eV, by the Supramolecular method is obviously deviated from the reported experimental value 4.46 eV. The corresponding value given by the two-layer ONIOM method, 4.43 eV, is in good agreement with the experimental value. (3) The optimized energy of the fluorescent emission state (S₁ state) are 3.61 and 3.87 eV by Supramolecular and ONIOM methods, respectively. The calculated oscillator strengths, in both gas and water clusters, were compared with reported experimental and theoretical results. These results indicated that both Supramolecular and ONIOM methods, combined with TD DFT B3LYP/6311++G(d), can provide good results of calculating excited state and spectra properties of 2Ap in condensed phase. This fact encouraged us to extend our study to 2Ap-T base pair and its solvated model so as to obtain the spectra properties of 2Ap in real DNA environment.